Computers are increasingly being connected to communication lines and other devices or networks with the computers performing as servers to the peripherally connected computers or devices. The data transfer throughput of computer servers can be increased significantly by using fiber optic lines.
An important component of many optical communication systems is the photo-detector which converts the optical signal into electric current. However, to overcome the degrading effect of circuit noise inherent in the electronic component of the receiver (in pre-amplifier stage), the conversion of light into electricity must be accompanied by amplification. A commonly used photo-detector that simultaneously detects light and internally amplifies the current is the Avalanche Photo Diode (APD). An APD is a solid state device (reverse-biased p-i-n junction) that can generate high gains. Each absorbed photon is converted into a photo-current pulse whose total area is a large multiple of the electronic charge. This gain factor, however, is inherently noisy: the net gain fluctuates each time a photon is absorbed.
Gain factor of photo-current of APD affects the receiving sensitivity. A high gain factor generates a relatively high shot noise and therefore decreases the detecting sensitivity of the system, whereas a low gain factor generates a low photo-current pulse below the optimal level of sensitivity. The level of the photo-current gain factor for an APD depends on the bias voltage applied to the APD. A higher bias voltage on an APD translates to a higher gain factor. The key to improve detecting sensitivity of the system is to the bias voltage on the APD such that the APD generates a high photo-current pulse while keeping the shot noise as low as possible.
Because an APD works with high internal fields, it can be sensitive to changes in the operating temperature. When holding the APD bias voltage constant, an increase in temperature will decrease the avalanche gain. A temperature compensation circuit on the APD bias voltage supply is typically used if the required operating temperature range is large enough to significantly impact on receiver performance.
One commonly used methodology to compensate the APD bias voltage is based on the generation and use of an informal formula, derived from pervious experiences. The formula is first used to estimate the temperature-to-bias-voltage characteristics at various temperature environments. The parameters of the compensation circuit are adjusted during testing. Due to individual difference of optical-electronic components, there is a big error to the estimation methodology. Since there are many parameters affecting the compensation circuit, this methodology with a single informal formula offers a hard adjustment and a low efficiency. Sometimes the APD even cannot achieve a high sensitivity after the bias voltage compensation based on this methodology.